Costimulatory B7 molecules (B7-1 or CD80; and B7-2 or CD86) are known to bind to T cell costimulatory receptors CD28 and CTLA4. Engagement of CD28 is know to transduce signals in T cells that play a critical role in T cell activation. TCR mediated activation of T cells to proliferation and IL2 secretion was enhanced in mice deficient in the adapter molecule cbl-b, suggesting a negative regulatory role for cbl-b under these conditions. Cbl-b deletion in fact reversed the inability of CD28-deficient mice to generate T cell-dependent Ig class switched primary and secondary antibody responses. The observation that cbl-b inactivation can enhance T cell responses to TCR signaling, and can bypass requirements for CD28 costimulation has been translated to studies of tumor rejection. Inactivation of cbl-b enhances the ability of mice to reject tumors that do not express costimulatory ligands (CD80 or CD86), and that grow progressively in wild type mice. Cbl-b inactivation also dramatically inhibits the incidence of thymic lymphomas in ATM-deficient mice. These findings identify a novel strategy for enhancing T cell-mediated tumor immunotherapy by modulating regulatory influences on T cell activation. The relationship between members of the cbl family and other critical adapter molecules has been studied. Experiments have indicated that inactivation of c-cbl (but not cbl-b) completely reverses the lethal phenotype caused by SLP-76 deficiency. In addition, c-cbl inactivation partially reverses the defect in T cell development caused by deficiency in SLP-76, LAT,or Vav1. These findings indicate an unanticipated SLP-76 (and LAT) independent signaling pathway that is facilitated by c-cbl inactivation. The biochemical basis for these effects has been studied using SLP-76 mutants to dissect the structure-function relationships in this pathway. C-cbl specifically rescues the T cell developmental defect resulting from SLP-76 mutations that disrupt tyrosines involved in binding to Vav and Itk. Subsequent analysis has further indicated that c-cbl inactivation rescues the developmental defect in Vav1-deficient mice, further characterizing a SLP-76/LAT/Vav1 pathway for T cell development. In contrast, c-cbl inactivation fails to overcome the T cell developmental defects that occur un lck-deficient mice. Thus, the pathways unmasked by c-cbl deficiency appear to require lck. The molecular components of this pathway are currently under study. The src family kinase lck plays a critical role in T lymphocyte development and activation. Expression of lck is regulated by two promoters, termed distal and proximal promoters, in both mice and humans. Although the existence of these promoters was desribed 20 years ago, their function in developing and mature T cels has not been analyzed. To address this question, we have generated mutant BAC transgenes by deletion of either proximal or distal lck promoter. Studies of these mice have demonstrated selective expression of lck at preferential and different stages of development when driven by one or the other of its two promoters. Corresponding slective stages of T cell development and differentiation have been shown to be dependent upon promoter-specific lck expression. We have made the unexpected observation that antigen-specific proliferative responses of naive and memory CD4 T cells require the down-modulation of tumor suppressor p53. In the absence of TCR signal, IL-2 induces a sustained increase in p53 protein, which prevents proliferative responses despite strong signaling through the IL-2 receptor. In contrast, TCR signaling results in early termination of p53 protein expression by decreasing p53 mRNA as well as by strong transcriptional induction of the p53-regulating protein Mdm2. Down-modulation of p53 in response to antigen stimulation is in fact critical for antigen-specific T cell proliferation; and preventing p53 degradation by inhibiting Mdm2 results in sustained p53 protein levels and prevents antigen-specific T cell proliferation. These studies elucidate a critical role of p53 as a negative regulator of T cell proliferation. It is the termination of p53 elevation by TCR signaling that allows proliferative responses to occur, enforcing antigen specificity.